This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-143466, filed May 24, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a radio communication system that is applied to, for example, indoor wireless radio LAN systems and adapted for data transmissions between a base station and a number of terminal stations.
Radio communications in indoor space are highly susceptible to fading due to multipath (multiple reflected waves). It is therefore effective to transmit and receive with directivity concentrated in the direction of maximum reception. For example, Japanese Unexamined Patent Publication No. 9-232848 discloses radio communications equipment that is constructed, as shown in FIG. 15, from an antenna array 1, circulators 2, receive modules 3 each comprising a low-noise amplifier and a down converter (D/C), analog-to-digital (A/D) converters 4, quasi-coherent detectors 5, local oscillators 6 and 7, a digital beam former (DBF) 8, a beam selector 9, a maximum ratio combiner 10, a demodulator 11, an in-phase divider 12, phase/amplitude correction units 13, a weight separation calculation circuit 14, a transmission weight calculation circuit 15, quadrature modulators 16, transmitting modules 17 each consisting of an up converter and a transmitting power amplifier, and transmitting local oscillators 18 and 19.
Signals received by the elements of the antenna array 1 are amplified and converted into intermediate-frequency signals in the receive modules 3 and then converted into digital signals in the AID converters 4. The digital signals are subjected to quasi-coherent detection in the quasi-coherent detectors 5 and then combined in the maximum ratio combination circuit 10 while varying their weight values so that S/N (signal to noise ratio) becomes maximum. At transmit time, the transmission weight calculation circuit 15 calculates transmitting weight values on the basis of the receiving weight values calculated by the weight separation/calculation circuit 14. After transmit signals are weighted by the transmitting weight values, a transmitting main beam is formed and transmitted in the direction of maximum reception from the antenna array 1 via the quadrature modulators 16 and the transmitting modules 17.
The conventional equipment needs to calculate the direction of arriving radiation in real time (namely, sequentially) and switch the directivity of the antenna in order to track the station with which the connection has been set up. As a result, the calculation processing is performed mainly by hardware rather than by software. Thus, the equipment has a problem of being complex in circuit arrangement and increasing in size.
To solve that problem, Japanese Unexamined Patent Publication No. 9-219615 discloses an adaptive array transmitter-receiver, which is provided, as shown in FIG. 16, with an array antenna 21 comprised of a plurality of antenna elements, weighting units 22 for weighting the amplitude and phase of each of signals which are to be transmitted from or are received by the antenna elements, a divider/combiner 23 for dividing transmit signals to the antenna elements or combining received signals from the antenna elements through the weighting units 22, an interface (I/F) 25, and an external operations unit 26. The external operations unit 26 performs calculations for adaptive control of the antenna directivity in non-real time for each terminal to which transmission is to be performed and makes weight switching from terminal to terminal on a time-division basis with a time slot preallocated for each terminal. This allows the transmitter-receiver to be made simple in construction and reduced in size.
Each weighting unit 22 varies the phase and the amplitude in a signal supplied from the dividing/synthesis units 23 by predetermined amounts, or multiply the signal by a complex weight. Thus, the radio waves radiated from the antenna elements form a desired synthetic transmission directional pattern.
On the other hand, the phase and the amplitude of each of the signals received by the plural antenna elements are controlled by the weighting units 22 or the signal is multiplied by the complex weight. Then, the signals are combined by the dividing/synthesis unit 23. The desired receiving directional pattern can be formed in this manner.
In the adaptive array transmitter-receiver disclosed in Japanese Unexamined Patent Publication No. 9-219615, in performing communications between the base station installed with the adaptive array antenna and each terminal, transmitting and receiving time slots are preallocated for each terminal. That is, with a system comprising a base station and N terminals, N transmitting time slots and N receiving time slots are preallocated. Each terminal is allowed to communicate with the base station only in time slots allocated to it.
With such a configuration, time slots allocated for each terminal are occupied by it regardless of whether it makes a request for communications. In the absence of a communication request, therefore, the allocated time slots result in waste of time. In addition, for terminals having a request for communications of large amounts of data, fixed time slots are not enough for such communication because of limited communication time. Data have therefore to be transmitted in a number of time slots, which results in reduced data communication efficiency.
In recent years, a wireless POS (point of sales) system has been installed in large-scale stores. In this wireless POS system, which consists of a host computer connected to the base station and a number of POS terminals connected with terminal station, inquiries about prices of goods and sales registration data are transmitted between the host computer and each POS terminal by radio. If, when the aforementioned adaptive array transmitter-receiver is used with the wireless POS system, communication requests for inquiries about prices of goods and sales registration data are intensively made by a certain POS terminal to the host computer, the length of the time slots allocated for that terminal may sometimes become insufficient. On the other hand, when another POS terminal makes no communication request, the time slots allocated for it result in waste of time. Therefore, the wireless POS system used with the conventional Adaptive array antenna is poor in efficiency.
It is an object of the present invention to provide a radio communications device for use in a base station which permits communications to be efficiently performed with a terminal station that makes a request for communications, does not spend any time on terminal stations that make no request for communications, and is simple in construction and a radio communication system which includes such a base station and a number of terminal stations.
In order to achieve the above object, according to the present invention, there is provided a radio communication system comprising a base station having a radio communications device that includes a receiving section which weights the amplitude and phase of each of received signals from a plurality of antenna elements, then combines the weighted signals into a composite signal and demodulates the resultant composite signal and a transmitting section which divides a modulated signal into a plurality of transmit signals, then weights the amplitude and phase of each of the transmit signals and radiates each of the weighted transmit signals from a corresponding respective one of the antenna elements, and a plurality of terminal stations each of which performs communications with the base station by radio, each of the terminal stations including transmitting means for, when a communication request occurs in it, transmitting a communication request signal in response to reception of a base station identification signal from the base station; and the base station further comprising: weight value storage means for storing a set of amplitude and phase weight values for each of the terminal stations, each set of amplitude and phase weight values being used when a corresponding one of the terminal stations communicates with the base station and the amplitude and phase weight values in each set being determined for each of the antenna elements, and storing scanning amplitude and phase weight values used when the base station scans its associated service area with directional beams to search for a terminal station that makes a request for communications; base station identification signal transmitting means for transmitting a base station identification signal via the transmitting section; communication request receiving means for receiving the communication request signal transmitted from a terminal station in response to the base station identification signal; terminal station identifying means for identifying the requesting terminal station that has transmitted the communication request signal received by the communication request receiving means; directivity control means for, when no terminal station to communicate with the base station is specified, controlling the directivity of the transmitting and receiving sections on the basis of the scanning amplitude and phase weight values to cause the base station to scan its associated service area with directional beams and, when a terminal station is specified, fixing the directivity of the transmitting and receiving sections on the basis of the weight values corresponding to that terminal station specified; and means for radio communicating with the terminal station specified with the directivity of the transmitting and receiving sections controlled by the directivity control means.
In the present invention, without allocating time slots for each terminal station, a terminal station that makes a request for communications is identified exactly and the directivity of the antenna for that terminal station is determined by preset weight values for communications between the base station and the identified terminal station. Thus, the present invention allows efficient communications with a terminal station that makes a request for communications and does not spend any time on terminal stations that make no request for communications. In addition, the weight values are not calculated for each terminal station in real time but are stored previously in a memory, thus providing a radio communication system simple in construction.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.